The labelling by [14C]amino acids of cell-sap protein in a cell-free system from guinea-pig liver. The site of origin of labelled protein

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Abstract
When a microsome-cell sap preparation from guinea pig liver is incubated with Mg ions and 3-phosphoglycerate, as an adenosine triphosphate generator, C14 amino acids are incorporated into the protein of both microsome and cell sap fractions. There is a 10-minute delay before appreciable label appears in the cell sap protein. There is no significant incorporation under a variety of conditions into the cell sap protein in the absence of microsome material. Microsome material labelled by incubation with C14 amino acids for 10 minutes, then reincubated with fresh unlabelled cell sap, gives an increase in specific activity of cell sap protein when supplemented with 3-phosphoglycerate and Mg ions; this is accompanied by an approximately equivalent decrease in radioactivity of microsome protein. This suggests a transfer of radioactivity from microsome to cell sap protein. In the absence of 3-phosphoglycerate, there is marked reduction in labelling of cell sap protein, indicating that an adenosine triphosphate-dependent reaction is associated with the transfer of radioactivity from microsome material. Absence of Mg ions also reduces the labelling of cell sap protein. Replacement of 75% of cell sap by 0.25 [image] sucrose reduces radioactivity transfer from labelled microsomes, despite supplementation with adenosine- and guanosine triphosphate. Microsome material labelled by incubation with C14 amino acids for 10 minutes can incorporate amino acids into its protein when resuspended in fresh cell sap, at a somewhat reduced rate. The labelled material in the cell sap appears to be soluble protein. The possible nature of this material is discussed.